1. Field of the Invention
This invention relates to a camshaft driving system for an internal combustion engine, and more particularly to a camshaft driving system for a DOHC (double-overhead-camshaft) engine having two rows of cylinders.
2. Description of the Prior Art
There has been known a DOHC engine having two rows of cylinders in which a pair of camshafts, one for driving the intake valves and the other for driving the exhaust valves, are provided in the cylinder head of each of the cylinder rows, and the cylinder rows are displaced from each other in the axial direction of the crankshaft so that a vacant space is formed in front of one of the cylinder rows and behind the other cylinder row.
In such an engine, a crank pulley on the crankshaft and a pair of timing pulleys fixed to the camshafts for the intake valves and the exhaust valves respectively are operatively connected with each other by a single timing belt. This arrangement is disadvantageous from the viewpoint of use of space especially in the direction of width of the engine in that, since the ratio of the rotational speed of the camshafts to that of the crankshaft is fixed (for example, in a fourcycle engine, the camshafts should make two revolutions per one revolution of the crankshaft) and the minimum diameter of the crank pulley is inherently determined by the surface pressure acting on the timing belt and the like, the diameter of the timing pulleys is determined, that is, reduction in the diameter of the timing pulleys is limited.
In Japanese Unexamined Patent Publication No. 54(1979)-50718, there is disclosed an engine in which an interlocking mechanism for connecting, to the crankshaft, the camshafts for driving the intake and exhaust valves in the rearwardly displaced cylinder row is disposed in the vacant space provided in front of the cylinder row and an interlocking mechanism for connecting, to the crankshaft, the camshafts for driving the intake and exhaust valves in the forwardly displaced cylinder row is disposed in the vacant space provided behind the cylinder row. However this approach cannot substantially contribute to reduction of the engine size since the interlocking mechanism is provided for each cylinder row and accordingly the overall length of the engine is increased.